1. Field of the Invention
The present invention relates to a method and apparatus for reproducing information by irradiating an optical recording medium with a reproduction light beam, and in particular to a method and apparatus for reproducing information using a magneto-optical recording medium of domain wall displacement detection type.
2. Related Background Art
Conventionally, a magneto-optical recording medium has been used for a rewritable high-density recording system in which information is recorded by writing magnetic domains in a magnetic thin film using the thermal energy of a semiconductor laser and from which the information is read using a magneto-optical effect. Also, in recent years, a demand has grown to further enhance the recording density of this magneto-optical recording medium to obtain a large-capacity recording medium.
By the way, the linear recording density of an optical disk, such as a magneto-optical recording medium, greatly depends on the laser wavelength of a reproduction optical system and the numerical aperture of an objective lens. That is, when the laser wavelength λ of the reproduction optical system and the numerical aperture NA of the objective lens are determined, the diameter of a beam waist is also determined. Hence, the detectable spatial frequency at the time of reproduction of recording marks has an upper limit of around 2 NA/λ.
Therefore, in order to increase the density of the conventional optical disk, it is necessary to shorten the laser wavelength of the reproduction optical system and to increase the NA of the objective lens. However, the improvement of the laser wavelength and the numerical aperture of the objective lens has limitations. Consequently, various techniques have been developed to improve the recording density by devising the construction of the recording medium and a reading method therefrom.
For instance, Japanese Patent Application Laid-Open No. H06-290496 proposes a signal reproduction method and apparatus for recording a signal in a recording layer that is a multilayered film including a reproduction layer and a recording layer magnetically connected to each other, and at the time of reproduction, displacing the domain wall of a recording mark of the reproduction layer without changing recording data in the recording layer by utilizing a temperature gradient caused by irradiation with a light beam for heating to thereby magnetize almost the entire region of the reproduction layer in the spot of the light beam for reproduction in the same direction, detecting the change of the deflecting surface of reflection light of the light beam for reproduction and reproducing the recording mark below the diffraction limit of light.
According to this method, a reproduction signal becomes a rectangular shape, so that it becomes possible to reproduce the recording mark below the diffraction limit of an optical system without lowering a reproduction signal amplitude, thereby largely improving the recording density and the transfer rate.
FIG. 6 is a block diagram showing a construction of a magneto-optical recording and reproduction apparatus using a magneto-optical recording medium of domain wall displacement detection type. In this figure, reference numeral 1 denotes a magneto-optical disk of domain wall displacement detection type. This magneto-optical disk 1 has a construction where a magneto-optical recording film 3 is formed on a substrate 2 made of glass or a plastic, and a protective film 4 is further formed on the magneto-optical recording film 3.
The magneto-optical recording film 3 is produced by stacking a plurality of magnetic layers, and information is reproduced from the film by displacing the domain wall of a recording mark of the reproduction layer and enlarging magnetization in a reproduction spot without changing recording data in the recording layer by utilizing a temperature gradient caused by irradiation with a light beam for reproduction in a manner to be described in detail later. Then, changing of the deflecting surface of reflection light of the light beam for reproduction by the medium is detected and the recording mark below the diffraction limit of an optical system is reproduced.
The magneto-optical disk 1 is supported above a spindle motor (not shown) by magnet chucking or the like and is set so as to be freely rotated with respect to a rotation axis. Reference numeral 5 denotes an optical head that irradiates the magneto-optical disk 1 with laser light for recording/reproduction and gains information from reflection light. This optical head 5 includes a condensing lens (for instance, NA: 0.60), an actuator that drives the condensing lens, a semiconductor laser (for instance, wavelength λ: 650 nm), a beam splitter, a polarizing beam splitter, and a photosensor that detects reflection light from the magneto-optical disk 1. The laser light emitted from the semiconductor laser is applied to the magneto-optical disk 1 through a group of optical components such as the condensing lens.
At this time, the condensing lens is controlled by the actuator so as to move in a focusing direction and a tracking direction in order to sequentially focus the laser light on the magneto-optical recording film 3 of the magneto-optical disk 1 and to perform tracking along a guide groove formed in the magneto-optical disk 1.
The laser light reflected from the magneto-optical disk 1 is detected by the photosensor through the group of optical components. The photosensor is divided into a plurality of sensors, with the laser light being deflected in different defection directions corresponding to the polarity of the magnetization of the magneto-optical recording film 3 and being condensed on the respective sensors corresponding to the different defection directions. Then, differential amplification of outputs from those sensors generates a magneto-optical signal. This magneto-optical signal is inputted into a partial response maximum likelihood (PRML) processing circuit through LPF (not shown) and the like, thereby gaining reproduction information with higher accuracy. In this specification, a PR equalizing circuit 11 for performing waveform equalization of a reproduction signal and a Viterbi decoding circuit 12 for performing Viterbi decoding are collectively referred to as the “PRML processing circuit”.
A controller 6 controls an LD driver 7, a magnetic head driver 8, and the like by inputting, input information, the number of rotations and recording radius of the magneto-optical disk 1 and recording sector information as well as the environmental temperature and the like; and outputting a recording power, a recording signal, and the like. Reference numeral 9 denotes a magnetic head that applies a modulation magnetic field to a laser irradiation region of the magneto-optical disk 1 at the time of a recording operation and is arranged so as to oppose the optical head 5 with the interposition of the magneto-optical disk 1 therebetween. The magnetic head 9 is driven by the magnetic head driver 8.
At the time of information recording, the optical head 5 irradiates the magneto-optical disk 1 with recording laser light and simultaneously a magnetic field having a different polarity is applied from the magnetic head 9 in accordance with a recording signal. Also, the magnetic head 9 moves in the radial direction of the magneto-optical disk 1 in an interlocked manner with the optical head 5 and sequentially applies the magnetic field to the laser light irradiation region of the magneto-optical disk 1 at the time of recording, thereby recording information.
Next, an operation for recording information will be described with reference to FIGS. 7A to 7E. FIG. 7A shows a recording signal, FIG. 7B shows a recording power of the semiconductor laser, FIG. 7C shows a modulation magnetic field of the magnetic head 9, FIG. 7D shows a recording mark string recorded in the magneto-optical disk 1, and FIG. 7E shows a reproduction signal.
At the time of recording of the recording signal as shown in FIG. 7A, a light beam for recording having the predetermined recording power as shown in FIG. 7B is applied from the optical head 5 to the magneto-optical disk 1 concurrently with the start of the recording operation, and the modulation magnetic field as shown in FIG. 7C based on the recording signal of FIG. 7A is simultaneously applied to a light beam irradiation region of the magneto-optical disk 1 from the magnetic head 9.
Here, so-called pulse magnetic field modulation is adopted in which the applied magnetic field is modulated in accordance with the recording information and the recording laser light is modulated in a pulse manner in stationary cycles. As a result of those operations, in the course of cooling of the magneto-optical recording film 3 of the magneto-optical disk 1, the recording mark string as shown in FIG. 7D is formed in the magneto-optical disk 1. The slanted line portions and open portions of the recording mark string as shown in FIG. 7D represent magnetic domains having mutually opposite magnetization directions. Also, adopting such magnetic field modulation recording enables magnetic domains smaller than a spot size to be formed.
Next, a reproduction operation will be described with reference to FIGS. 8A to 8D. In the following description, a case will be described as an example, in which the magneto-optical recording film 3 of the magneto-optical disk 1 has a three-layer structure composed of a record holding layer (recording layer) where recording marks are held; a domain wall displacement layer (reproduction layer) where a domain wall is displaced to directly contribute to a reproduction signal; and a switching layer that switches a connection state between the recording layer and the reproduction layer.
FIG. 8A shows the state of magnetic domain reproduction, FIG. 8B shows a recording film state, FIG. 8C shows a medium temperature state, and FIG. 8D shows a reproduction signal. At the time of information reproduction, as shown in FIG. 8A, the magneto-optical recording film 3 is heated to a Ts temperature condition at which a domain wall of the reproduction layer of the magneto-optical recording film 3 is displaced, by irradiation with a light beam for reproduction. At this time, the switching layer as shown in FIG. 8B is placed under a state where it is connected with the recording layer and the reproduction layer by an exchange coupling in a region having a temperature below Ts.
When the magneto-optical recording film 3 is heated to the temperature Ts or higher by the irradiation with the light beam, the temperature of the switching layer reaches a Curie point and the switching layer is placed under a state where the connection with the reproduction layer and the recording layer is disconnected. Therefore, concurrently with the reaching of the domain wall of a recording mark to this Ts temperature range, the domain wall of the reproduction layer is instantly displaced to a position at which the domain wall exists with stability in terms of energy with respect to a temperature gradient of the reproduction layer, that is, to the maximum temperature point in the linear density direction of a temperature rise due to the irradiation with the light beam, while going across a land.
As a result, the magnetization state of the great majority of a region covered with the light beam for reproduction becomes the same, so that it becomes possible to obtain a reproduction signal having a shape close to the rectangular shape as shown in FIG. 8D even from a minute recording mark that cannot be reproduced according to an ordinary light beam reproduction principle. Accordingly, it becomes possible to reproduce the recording mark below the diffraction limit of an optical system with almost no lowering of a reproduction signal amplitude and to substantially improve a recording density and a transfer rate.
Next, a reproduction signal processing method of the PRML processing circuit shown in FIG. 6 will be described. In partial response signal processing applied to an ordinary optical disk reproduction signal, a partial response PR (1,1) or PR (1,2,1) is generally used because the optical disk reproduction signal has DC components and inter-code interferences due to lowering of a spatial frequency resulting from limitations of the optical system exert an influence.
When a magneto-optical medium of domain wall displacement detection type is used, however, the inter-code interferences due to the limitations of the optical system do not occur and a merit in adopting partial response that is a differentiating system is advocated against the backdrop of an increase in influence of crosstalk resulting from a reduction in track pitch and the like. For instance, by adopting a partial response PR (1, −1), it becomes possible to perform reproduction signal processing where the influence of low-frequency crosstalk components that are optically readable spatial frequencies is eliminated.
FIGS. 9A to 9H illustrate an example of PR (1, −1) reproduction signal processing. Note that a case where a (1, 7) RLL code is used as a modulation code will be described as an example. FIG. 9A shows an information signal string, FIG. 9B shows a recording signal string, FIG. 9C shows a recording mark string in the case where the recording signal is recorded in the magneto-optical disk, FIG. 9D shows a magneto-optical reproduction signal of domain wall displacement detection type in the case where the recording marks are reproduced, FIG. 9E shows a sampling signal level in the case where the reproduction signal of FIG. 9D is sampled with reference to PLL clocks, and FIG. 9F shows a signal level in the case where PR (1, −1) equalizing processing is performed on the sampling data of FIG. 9E.
FIG. 9G also shows a histogram of the signal level of FIG. 9F, that is, so-called level jitter. FIG. 9H shows a result of PR reproduction processing judgment where a three-value-level judgment is performed on the signal level of FIG. 9F and it is judged whether the signal level is “0” or not. In more detail, when the signal level is “0”, a judged result gives “0”, and, when the signal level is not “0”, a judged result gives “1”.
Here, the information string as shown in FIG. 9H is reproduction of the information signal string as shown in FIG. 9A, which means that it is possible to reproduce the information. The fundamental processing principle of the PR processing has been described above. However, usually the three-value-level judgment as shown in FIG. 9H is not directly performed, a technique is used in which the signal level of FIG. 9F is further processed and subjected to PRML processing which performs reproduction signal processing using maximum likelihood decoding (ML) or Viterbi decoding, whereby reproduction information with less errors is obtained.
In conventional magneto-optical reproduction of domain wall displacement detection reproduction type, the reproduction signal waveform becomes a rectangular waveform having no inter-code interference, as described above. FIG. 10A shows an example of this reproduction signal waveform. As shown in FIG. 10A, when attention is focused on a saturation amplitude portion (portion surrounded by a circle A) where no level fluctuation of the signal waveform originally exists, many high-frequency noise components are actually contained. Also, in a portion (portion surrounded by a circle B) where no level fluctuation originally exists, there sometimes occurs a situation where a level fluctuation and a waveform distortion occur.
Those high-frequency noise and waveform distortion lead to a level error at the time of PR detection, that is, those become a cause for deterioration of level jitter in the signal level histogram shown in FIG. 9G. In ordinary cases, those noises are removed using a low-pass filter (LPF). However, the LPF affects not only the noises but also the inclinations of the rising edge and falling edge of signal polarity changing of the rectangular reproduction signal. Consequently, there is a case where the excess application of the LPF results in deterioration of jitter at the edge positions of the signal polarity changing and an adverse effect is exerted on PLL detection and the like.
FIG. 10B also shows a sampling level in the case where sampling is performed after the reproduction signal of FIG. 10A has passed through the LPF. As can be seen from this figure, the rising edge and falling edge of the signal polarity changing are gently changed. If the rising edge and the falling edge are changed in this manner, this leads to a problem in that levels of the reproduction signal at sampling points with interposition of the edge position therebetween are changed as indicated by arrows in FIG. 10B and this in turn changes levels in the histogram.